In many modern electronic devices a cooling fan is used to regulate the temperature of the device during operation. In most laser printers, this is an especially important design element due to the presence of an active heating element, typically a fuser that melts the toner to the paper. In such equipment, the cooling fan must be controlled in such a way so as to strike a suitable balance between two competing goals of controlling the internal temperature of the device and minimizing the acoustic impact of the device upon its environment. The designer can always install a fan that blows more air, but this approach quite often causes a greater noise level that usually is undesirable.
In laser printers, for example, it is common practice to operate an internal cooling fan at more than one speed, tailoring the fan's operation to the needs of the printer in each of its operating modes. For example, one might operate the fan at a high speed during active printing operations to ensure maximum cooling efficiency, but then change to a slower (and quieter) speed during an “idle” mode, and perhaps turn the fan off for a “power saver” mode. The nominal operating speed of the fan in each mode typically is chosen to ensure adequate cooling under worst-case variations of printer temperature and variations in fan operating characteristics.
Cooling fans, like all manufactured devices, may exhibit significant variation from unit to unit in their response to control stimulus. For example, one particular fan may run at 2400 RPM in response to a given control signal, and another fan of the same design may run at 2500 RPM, given the same control signal input. This variation in operating response is generally outlined in the published operating specifications for the fan unit, as tested and reported by the fan manufacturer. The system designer who desires a particular nominal operating speed (or speeds if there is more than one operating mode) of the fan within a device must take into consideration these variations in control response in order to ensure proper operation of the fan. These variations may be amplified by the potentially nonlinear response of the fan to the host device's control circuit, and by manufacturing variances in the control circuit itself. Any nominal operating speed chosen must also account for these variations.
Many fans commonly available provide a feedback signal to the host device which gives an indication both that the fan is indeed running and not stalled, and that the fan is running at a particular RPM speed. This type of feedback signal, when available in a particular fan, will be referred to herein as a “tach signal” or a “tachometer signal.” A common embodiment of this type of tach signal in conventional low-cost fans is a simple-buffered feedback of one of the coil switching outputs of the fan's internal motor controller. Such a feedback signal would appear as a pulse train, similar to a “clock” signal, at a frequency on the order of two-times to four-times (2×–4×) the fan's RPM, depending on the number of poles in the fan motor and other motor design characteristics.
Many conventional fans used in printers typically include internal motor control logic that maintains the fan motor at a stable, consistent speed given its external control input. If available, the “tach” feedback signal provided to the controlling system is usually intended as a means of monitoring the fan's operation, but not necessarily as a means of actively controlling the fan's speed. The frequency of the “tach” signal in these low cost fans is generally too low to provide enough information for a host device to use conventional motor control means for actively controlling the speed of the fan.
In applications where operational consistency in either acoustics or precise fan RPM (or both) is of concern, it is desirable for the host device to have some means for controlling the fan RPM in such a way that manufacturing and environmental variations from fan to fan, as well as nonlinearity of the fan's control response, are minimized or eliminated. Since the typical feedback signal from low-cost fans has been inappropriate for traditional control means, it would be an improvement to provide a control circuit and operating method that would allow suitable control of such fans while using the low-frequency tach feedback signal that is commonly available in such fans.